Diode bridges, either full or half bridges, are very common circuit elements to perform rectification of oscillating output signal. A half bridge is made of two diodes and has three external electrodes. They are commonly used for the output rectification in switched-mode power supply circuits: e.g. in forward, push-pull, half bridge and full bridge topologies. For one polarity of the signal, electric current flows through one diode (in ON state) and not through the other (in OFF state). For the opposite polarity the diodes switch their states: the ON diode goes to OFF state and the OFF diode changes to ON state. This switching of the current flow between the two diodes results in rectification.
The energy loss during signal rectification is determined by the performance of individual diode. For the real diode implementation, it is limited by the ideal diode equation:IF/IR≦exp(qVF/kT)
where IF is a forward current, VF is the forward bias voltage, IR is the leakage current and kT/q=0.0259V at room temperature T. Thus a certain rectification ratio requires that the forward voltage drop is larger than some limitVF>00259 ln(1+IF/IR)
For example, for a diode conducting a current of 10 A, with 10 μA leakage current, the forward voltage drop is larger than 0.358. Some diodes are close to this theoretical limit, leaving very small space for improvement through conventional techniques. This theoretical limit leads to very inefficient diodes once the electronics moves to lower voltages. For example, if a power supply uses 3.3V the losses on the half bridge will be about 0.358/3.3=11%. This high energy loss just for the rectification is unacceptable for modern switched-mode power supplies.
To overcome the high losses on rectifier bridges for low voltage applications, synchronous rectification is used. One approach is to use a MOSFET to perform the rectification function of the diode. However, the circuit implementation of synchronous rectification becomes very complicated. A controller is needed to provide the gate voltage and to change MOSFET from the ON to the OFF state. Sensors are needed to tell the controller that the sign of the applied voltage has changed. This additional signal processing reduces the speed of operation for half bridges made of synchronous rectifiers. Thus instead of 2 diodes, one needs a much more complicated and expensive circuit.
Thus, there has been a long-felt need for a bridge rectifier which can operate efficiently at low voltages, but without the complex circuitry and limited frequency range of conventional synchronous rectifiers.